1. Field of the Invention
The present invention relates to an electrophotographic process capable of forming an overlay image from a first image and a second image.
2. Description of the Prior Art
In the field of electrophotography there are already known various systems such as electrofax system, xerox system, P.I.P. (persistent internal polarization) system and NP system which is disclosed for example in the U.S. Pat. Nos. 3,666,363 and 4,071,361.
The electrofax and xerox systems depends upon so-called Carlson process as described in the U.S. Pat No. 2,297,691 wherein a layer of a photoconductive material such as zinc oxide (for electrofax system) or amorphous selenium (for xerox system) provided on a substrate is uniformly charged with a corona discharge and then subjected to an imagewise exposure to light to dissipate the charge in the exposed area, and an electrostatic image thus obtained corresponding to the original pattern is rendered visible by development with charged colored particles and fixed on said layer or after transfer onto another support material such as a paper sheet to obtain an electrophotographic image. The P.I.P. system utilizes the physical properties, i.e. persistent internal polarization for forming a latent image. Also the NP system utilizes the photoconductivity and electrostatic capacitance of a photoconductive layer and an insulating layer provided thereon for forming an electrostatic latent image, which is subsequently subjected to the steps of development, transfer and fixing in a similar manner to obtain an electrophotographic image.
Although various copying apparatus have been developed on these electrophotographic processes, such apparatus have been unable to satisfactorily meet, except for a limited extent, the increasing demand for exposing plural different originals onto a photosensitive member to obtain an overlaid latent image and thus to obtain an overlaid visible image. Also the prior image overlay processes principally depend on optical overlay of plural images or on electrically overlaid signals, and there have been known very few overlay processes for forming an overlay image on a photosensitive member by combining different steps constituting an electrophotographic process.
An example of such known overlay process utilizing the above-mentioned NP system is disclosed in the U.S. Pat. No. 4,122,462 assigned to the assignee of the present application. The present invention is to provide another overlay process and an apparatus adapted therefor.